1. Field of the Invention
The present invention relates to a start-up method for a fuel cell system that can improve the start-up performance.
2. Description of Related Art
A fuel cell is known in which an anode and a cathode are provided that have a solid polymer electrolyte membrane interposed therebetween, a fuel gas (for example, hydrogen gas) is supplied to the anode, an oxidizing gas (for example, air) is supplied to the cathode, and chemical energy that is produced during the oxidation-reduction reaction between these reacting gases is extracted directly as electrical energy.
In such a fuel cell, generally in order to increase the fuel utilization rate and thereby improve the fuel economy, a fuel circulation path is formed so that an unreacted hydrogen gas, i.e., a hydrogen gas that has not been consumed, that is extracted from the fuel cell is recycled, mixed with fresh fuel gas, and supplied again to the fuel cell.
A technology is known in which, when stopping these fuel cells, the supply of hydrogen gas is stopped after the supply of air is stopped, and thereby power generation action does not occur during the stoppage due to the reacting gases that remain in the fuel cells after the stoppage (for example, refer to Published Japanese Translation No. 2000-512069 of the PCT International Application).
However, even though the power supply to the external load is cut off during the stoppage of the fuel cell, reacting gases remaining in the system are consumed due to the oxidizing reaction caused by the catalysis of the anode and cathode. That is, air remains in the oxidizing agent supply system and hydrogen remains in the fuel circulation system, and these residual reacting components are gradually consumed depending on the duration of the fuel cell stoppage. However, although the oxygen gas in the air is consumed, the nitrogen gas remains in the system. Thus, the partial pressure of the nitrogen gas on the cathode side increases and the pressure in the hydrogen circulation system decreases. Thereby, the nitrogen gas passes through the solid polymer electrolyte membrane to leak to the anode side as well, and penetrates into the circulation system of the fuel cell.
However, as described above, when the nitrogen gas penetrates into the circulation system of the fuel cell, during start-up, when the reacting gases, and in particular hydrogen gas, are supplied, a problem is encountered in that the reaction does not occur smoothly when the reacting gases are supplied into the fuel cells due to the interference of the nitrogen gas present in the hydrogen circulation system.
In particular, in the case in which the fuel cell is for a vehicle, when nitrogen gas in the air penetrates into the system as described above, the start-up of the fuel cell takes time, and thus there are the problems that the starting performance of the fuel cell vehicle is degraded and the marketability thereof is reduced. In addition, depending on how the atmospheric pressure fluctuates with the passage of time after stoppage of the fuel cell system, contrariwise, the case in which the pressure in the anode falls below atmospheric pressure must also be considered. When the pressure falls below atmospheric pressure, there are cases in which impure gases flow back to the anode side, and this interferes with the smooth starting of the fuel cell.